In continuous ink jet printing, ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. The approach for printing with these droplet streams is to selectively charge and deflect certain drops from their normal trajectories. Graphic reproduction is accomplished by selectively charging and deflecting drops from the drop streams and depositing at least some of the drops on a print receiving medium while other of the drops strike a drop catcher device. The continuous stream ink jet printing process is described, for example, in U.S. Pat. Nos. 4,255,754; 4,698,123 and 4,751,517, the disclosures of each of which are totally incorporated herein by reference.
Achieving improvement in permanence and darkness of ink jet print remains a top priority in the printing business. For ink jet business to grow in the graphic arts, textiles, lottery, business forms and publishing industries, significant improvement in water resistance and darkness of the image must be demonstrated.
Substrate manufacturers produce many ink-jet grade substrates using coated components. Coated components, such as amorphous and precipitated silicas, vary in particle sizes and their distribution. Multivalent cross linking, often cationic, components include specialty treated high-bulk oxides of metals such as aluminum, titanium, zirconium, lanthanum, magnesium; modified starches; water soluble polymeric binders, such as hydrolyzed PVA; and resins. Depending on the composition and methods of coating, such substrates yield varying degrees of dot gain, brightness, optical density, rate of absorption or drying, and water resistance of the ink-jetted image.
The use of hydroxyalkylated polyethyleneimine (EPI) in ink jet inks has been shown to significantly improve waterfastness of the inks. However, polymeric inks using EPI or other polymers to improve permanence of an image, do not run in the printer as well as non-polymeric inks, and require more frequent system maintenance. Furthermore, non-black inks achieve only modest improvement in water resistance with EPI, probably due to extreme solubility of the non-black color dyes.
A coating fluid comprising a solution of ethoxylated polyethyleneimine, a surfactant and an electrolyte, has been described and claimed in commonly assigned, co-pending, application Ser. No. 09/572,550. The fluid, when applied with an ink jet printhead to an imaged substrate, showed significant improvement in waterfastness even in the case of non-black images which are normally hard to fix due to their small molecular weight and extreme solubility.
While the coating fluid described and claimed in U.S. Pat. application Ser. No. 09/572,550, is particularly well suited in many ink jet applications, the coating fluid unfortunately has the consistency of water (viscosity˜1.0 centipoise). Therefore, when the coating fluid is applied in a roller coater, results were not encouraging. The coating was heavy on the margins of the substrate, while the middle area was blotchy and non-uniform. This produced uneven images with poor waterfastness.
It is seen then that there is a need for an improved technique for achieving proper viscosity of the coating fluid for suitability in a roller application and finding a means to detect unevenness on a treated substrate.